Lithium metal is considered the ultimate anode material for achieving high energy density of batteries because it has the highest specific capacity (3860 mAh g−1, 2061 mAh cm−3), and the lowest electrochemical potential (−3.04 V vs. standard hydrogen electrode). However, critical challenges including lithium dendrite growth and limited coulombic efficiency (CE) during cycling have inhibited the practical use of lithium batteries in rechargeable batteries (Xu et al., Energy Environ. Sci. 2014, 7:513-537). Recently, research efforts have revived to improve the stability of Li metal anodes, aiming to realize the practical applications of Li metal batteries. Representative approaches include the use of electrolyte additives, superconcentrated electrolytes, polymeric or solid-state electrolytes, and selective/guided Li deposition.
Recently, superconcentrated electrolytes have received intensive research attention for Li metal batteries owing to their various unique functionalities. One particular study by Qian et al. reported that the use of superconcentrated electrolytes composed of single bis(fluorosulfonyl)imide (LiFSI) salt in dimethoxyethane (DME) could enable the high-rate cycling of a Li metal anode with high CEs (up to 99.1%) without dendrite formation (Qian et al., Nat. Commun. 2015, 6:6362). However, superconcentrated electrolytes usually suffer from high viscosity, and/or poor wetting of the separator and the cathode. It is not practically feasible to inject a highly viscous electrolyte into large-format lithium ion batteries, such as prismatic or cylindrically shaped batteries, during the manufacturing process. Furthermore, the high cost of the high concentration lithium salt also hinders the practical use of superconcentrated electrolytes.